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36697_Ward's World+MGH Black Hole

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Black Hole (continued) Gravitational-wave Detector (LIGO), opened a new and fruitful way of studying black holes, and revealed a mass range of stel- lar black holes greater than had been thought to be possible. Black hole classes In general, physicists recognize two broad classes of black holes, categorized by their masses. Stellar black holes Stellar black holes form from supergiant stars with an initial mass of approximately 10 times the mass of the Sun (solar masses) or more. When they reach the end of their lives, these colossal stars explode as supernovae, with a remaining core of approximately 3 solar masses collapsing into a black hole. Stars with a bit less initial mass collapse at the end of their lives into neutron stars that, although highly dense, do not pack as much matter into as small a volume as a black hole, and thus do not gravitationally create event horizons. Typical "dwarf " stars like the Sun evolve into remnants known as white dwarf stars. A stellar mass black hole can possess anywhere from a few to several dozens of solar masses. Supermassive black holes Supermassive black holes, found in the cores of nearly all galax- ies, can contain anywhere from millions to billions of times the Sun's mass. The formation of supermassive black holes remains speculative. Hypotheses include the direct collapse of ex- tremely massive gas clouds in the early universe into a likewise extremely massive "seed" black hole, which then voraciously consumed matter and merged with other black holes. This hypothesis explains how supermassive black holes could have formed, as they evidently have, within the relatively short time cosmic period of the first billion years following the big bang. Another hypothesis in this vein involves the formation of super- massive stars with tens of thousands of times the mass of the Sun, which then collapsed into supermassive black hole seeds. Observation Observations of black holes—which, by definition, are not directly detectable—rely on indirect methods using the gravi- tational interaction of the black hole with its surroundings. The first successful detection of a black hole in this manner involved the binary x-ray system Cygnus X-1 in 1971. These binary systems are composed of a massive, very dense object— a black hole or a neutron star—and a companion star. Because of strong gravitational forces, the companion star loses mass, which settles down into an accretion disk around the compact object (Fig. 2). Frictional forces cause the matter in the disk to spiral inward and also heat up the disk, causing it to emit thermal x-rays. Whether the compact object is a neutron star or a black hole can be determined by analyzing the mutual motion of the components of the binary system and calculating the mass of the compact body. If the compact object (known to be com- pact because it is too dim to shine as brightly as a normal star) has more than about 5 solar masses, it must be a black hole. In the notable case of Cygnus X-1, the mass determination gives a value of about 10 solar masses, a value much larger than the maximum mass for a gravitationally stable neutron star and therefore strong evidence for the existence of a black hole in this system. Another line of observational evidence for the existence of, in this case, supermassive black holes is drawn from active galac- tic nuclei (AGNs), whose luminosities are significantly higher than those of the largest nonactive galaxies. Variations in the luminosities of AGNs on time scales of tens of hours indicate a central engine whose diameter should be of the order of the product of the variation time scale and the speed of light. To generate observed AGN luminosities in a spherical volume defined by this diameter by nuclear-fusion processes like those taking place in stars, one would need such large quantities of matter that the gravitational potential energy would far exceed the nuclear energy. Therefore, the origin of the luminosity is attributed to the transformation of potential energy into radia- tion by the supermassive black hole as it devours matter from an accretion disk. Twin jets of material moving at near-light- speed blast outward from the black hole's poles as well; such astrophysical jets are commonly created by spinning objects, even much less massive ones such as protostars. The most lumi- nous AGN are known as quasars; AGN whose jets are pointed directly at Earth are known as blazars. + ward ' s science Fig. 2 Artist's impression of Cygnus X-1, a binary x-ray system located about 6,000 light-years from Earth and the first widely accepted astronomical discovery of a black hole. A hot, massive star (at left) is having mass drawn from it by the gravitational pull of a black hole (at right). The matter spirals around the black hole as an accretion disk. Friction within the disk causes its contents to heat up and emit high-energy light as x-rays. [Credit: NASA, ESA, Martin Kornmesser (ESA/Hubble)]

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